A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus are conventionally used, for example, in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures.
It is desirable to reduce the size of features in a lithographic pattern because this allows for a greater density of features on a given substrate area. In photolithography, the increased resolution may be achieved by using radiation of shorter wavelength. However, a significant decrease in wavelength augments concern about diffraction limits and the transparency of materials (e.g. lenses), often requiring complex and expensive constructions and materials.
An alternative for printing small features comprises transferring a pattern to a substrate by imprinting a pattern into an imprintable medium using a physical mould or template. The imprintable medium may be the substrate or a material coated on to a surface of the substrate. The imprintable medium may be functional or may be used as a “mask” to transfer a pattern to an underlying surface. The imprintable medium may for instance be provided as an imprintable material deposited on a substrate, such as a semiconductor material, to which the pattern defined by the template is to be transferred. Imprint lithography is thus essentially a moulding process on a micrometer or nanometer scale in which the topography of a template defines the pattern created on a substrate. Patterns may be layered as with optical lithography processes so that, in principle, imprint lithography could be used for such applications as IC manufacture.
Imprint lithography does not suffer from the use of very short wavelengths and resolution of the imprint pattern is believed to be primarily limited only by the resolution of the template fabrication. For instance, imprint lithography has been used to produce features in the sub-50 nm range with good resolution and line edge roughness compared to that achievable with conventional optical lithography processes.
In various imprint lithography processes, a layer of an imprintable medium is coated onto the substrate. For example, the imprintable medium may be spin coated onto the substrate. However, by using spin coating, it may be difficult to adjust the amount of imprintable material for the template pattern. For instance, it may be desirable to provide more imprintable medium in areas where the template pattern has a relatively low volume of protrusions and less imprintable material where the template pattern has a relatively high level of protrusions. Additionally, if a large area of the substrate is covered with the spin coating, the chance of a part of the imprintable medium curing at a different rate to another part may be increased. For example, in imprint lithography processes where UV radiation is used to cure the imprintable medium, spurious UV radiation may cause the imprintable medium to become cured before a pattern has been imprinted into it. A proposed solution is the use of printing (e.g. ink-jet printing, bubble jet printing, etc., or any printing method which can eject droplets of fluid) to deposit the imprintable medium (or any layer which is to be deposited) onto the substrate. By using (for example) ink-jet printing techniques, small droplets of imprintable medium can be used to cover the area of a substrate to which a pattern is to be imprinted and the number of droplets in a particular area can be adjusted to the amount of imprintable material desired in a particular area.